It is well known that ethers may be prepared by reacting an alcohol with an olefin to form the desired product. The reaction mixture containing catalysts and/or condensing components may be separated and further treated to permit attainment of the desired product specification.
MTBE is being used as a blending component in high-octane gasoline, as the gasoline additives based on lead and manganese have been phased out. Currently, all commercial processes for the manufacture of MTBE are based upon the liquid phase reaction of isobutene and methanol catalyzed by cationic ion-exchange resin (see: Izquierdo, J. F., Cunill, F., Vila M., Tejero J. and Tborra M. Equilibrium constants for methyl tertiary butyl ether liquid-phase synthesis. Journal of Chemical and Engineering Data, 1992, vol. 37, p. 339.; Brockwell, H. L., Sarathy P. R. and Trotta R. Synthesize ethers. Hydrocarbon Processing, 1991, vol. 70, No. 9, p. 133.; Chemical Economics Handbook, Gasoline Octane Improvers. CEH Marketing Report, 1986, p. 543, Stanford Research Institute, SRI International, Menlo Park, Calif.). The isobutene is obtained by the fluid catalytic cracking process, from the isomerization of n-butene and dehydrogenation of isobutane. The methanol is produced from syngas (a mixture of carbon monoxide and hydrogen) obtained from the steam reforming of natural gas. The cationic ion-exchange resins used in MTBE synthesis normally have the sulfonic acid functionality (see: Tejero, J. Journal of Molecular Catalysis, 1987, vol. 42, p. 257; Subraminium and Bhatia, Canadian Journal of Chemical Engineering, 1987, vol. 65, p. 613).
(CH3)2Cxe2x95x90CH2+CH3OHxe2x86x92(CH3)3Cxe2x80x94Oxe2x80x94CH3
These cationic ion-exchange resins are generally based on polystyrene-divinylbenzene backbone and have a very limited stability range with regard to operating temperatures, with temperatures above 100xc2x0 C., normally leading to irreversible destruction of the resin and loss of catalytic activity. The catalyst life in commercial operation is about two years. The MTBE synthesis reaction is exothermic, yielding xe2x88x9237.7 kJ molxe2x88x921 of energy.
The detrimental effects of instability of the resin catalyst used in the preparation of MTBE have been discussed in a patent by Takezono and Fujiwara [U.S. Pat. No. 4,182,913]. According to the findings of this study, at higher temperatures, a large quantity of acids is effused from the strongly acidic cation-exchange resin and the deterioration of the catalyst resin is accelerated. Even when the temperature is low, a small quantity of the strong acidic substance is effused into the reaction mixture. When such a reaction mixture containing the acid substance is fed into the succeeding step of unreacted gas separation, so as to separate the unreacted gas by distillation, the decomposition or reverse reaction of the main product is caused to occur, which reduces the yield. In addition, various portions of the apparatus are corroded by the strong acids released. In this reaction, diisobutene and tertiary butyl alcohol (TBA) are the by-products of dimerization of isobutene and reaction of water with isobutene respectively. The amount of diisobutene formed increases with rise in temperature [Chu and Kohl, Industrial and Engineering Chemistry Research, 1987, vol 26, p. 365]. TBA formation is insignificant as long as the feedstocks are thoroughly dried. According to LeChatlier""s principle, the reaction equilibrium for MTBE formation is more favorable at lower temperatures, but reaction rate is decreased considerably. Thus current operation temperatures appear to be limited by three factors: (i). resin catalyst instability at temperature above 100xc2x0 C.; (ii). poor selectivity due to dimerization above 100xc2x0 C.; and (iii). equilibrium conversion limitation. It is also pointed out that the progress of the reaction over cationic ion-exchange resin is usually complicated by various adsorption and diffusion factors, by swelling phenomenon, and by the variable distribution of the components between the solution and cationic exchanger phase.
The zeolites have a porous structure and are represented by the following general formula;
M2/nO.xAl2O3.ySiO2.zH2O
Where M is an alkali metal or alkaline earth metal cation or organic base cation, n is the valence of the cation and x and y are variables.
In U.S. Pat. No. 5,157,162 to Knifton there is disclosed a process for one-step synthesis of MTBE using tertiary butanol and methanol over a catalyst comprising fluorosulfonic acid modified montmorillonite clay at temperature of about 20xc2x0 C. to about 250xc2x0 C.
In U.S. Pat. No. 5,220,078, a process is disclosed for producing MTBE using tertiary butanol and methanol over a catalyst comprising of fluorophosphoric acid modified Y-type zeolite at temperature of about 20xc2x0 C. to about 250xc2x0 C.
U.S. Pat. No. 5,300,697 discloses a process for producing MTBE using tertiary butanol and methanol over a catalyst comprising of hydrogen fluoride modified Y-type zeolite at temperature of about 20xc2x0 C. to about 250xc2x0 C. All of these processes are limited by the fact that the conversion is low and the selectivity of the reaction for MTBE is quite small.
It would be a substantial improvement in the art if MTBE could be selectively synthesized from isobutene and methanol using a catalyst which allows for rapid conversion of isobutene. In our invention, aluminum fluoride-modified zeolite can be used as an improved and novel catalyst for the selective synthesis of MTBE from isobutene and methanol with high conversion. The accompanying examples demonstrate good yields of MTBE when using the modified zeolites of the instant invention for such a reaction.
In accordance with certain of its aspects, the novel method of this invention for preparing MTBE from methanol and isobutene comprises reacting methanol and isobutene in the presence of a catalyst comprising an aluminum fluoride-modified zeolite containing at least one metal from Group IIIA of the periodic table. Examples demonstrate particularly the effectiveness of an aluminum fluoride-modified MFI-type zeolite. MFI is the structure type code (allocated by the Structure Commission of the International Zeolite Association) to a number of zeolites having similar topology such as ZSM-5 and silicalite.
Preparation of the product using this invention may be carried out typically by reacting methanol and isobutene in the presence of an etherification catalyst. The etherification is carried out in one-step and the catalyst preferably comprises MFI-type zeolite modified hydrothermally with aluminum fluoride. This important reaction does not restrict the scope of the invention.
The reaction of isobutene and methanol can be represented by the following equation: 
Generally the methanol and isobutene coreactants may be mixed in any proportion in order to generate the desired MTBE, but preferably the molar ratio of methanol to isobutene in the reaction mixture should be about 0.1 to 10. In order to achieve maximum selectivity to MTBE and optimum conversion per hour, an excess of methanol in the reaction mixture is desirable. The most preferred methanol-to-isobutene molar ratio is from 1:1 to 5:1.
The synthesis of MTBE according to the reaction given above can also be conducted where the isobutene and methanol reactants are mixed with other C4 aliphatic and olefinic hydrocarbons such as isobutane, n-butane and n-butene.
The same etherification process may also be applied for the preparation of other alkyl tertiary ethers. For example, the said etherification process may be applied to the reaction of a C1-C4 primary alcohol such as methanol, ethanol, n-propanol and n-butanol with a C4-C6 tertiary olefin, such as for example, tertiary amyl olefin. Reaction of methanol with tertiary amyl olefin would then yield methyl tertiary amyl methyl ether (TAME). Similarly, reactions of ethanol with isobutene would then yield ethyl tertiary butyl ether (ETBE).
Good results were realized using certain crystalline aluminosilicate zeolites as catalysts for the reaction of isobutene and methanol to produce MTBE. The preferred zeolites are the MFI-type zeolites as well as zeolite beta and mordenite, modified hydrothermally with aluminum fluoride.
Zeolites possesses a number of catalytically-favorable properties such as well-defined crystalline structure, uniform pores, high surface area, good thermal stability, wide range of acidity and shape selectivity.
The unit cell of a MFI-type ZSM-5 zeolite contains 96 silicon or aluminum-centered oxygen tetrahedra which are linked through shared oxygen atoms. Because of the net negative charge on each of the aluminum-centered tetrahedra, each unit cell contains an equivalent number of charge balancing cations. These are exclusively sodium ions in zeolites in their synthesized form. MFI-type ZSM-5 zeolite in its hydrated and sodium form has the following general formula in which the number of aluminum atoms should be less than 27.
Nan.AlnSi96-n.O192.16H2O
Particularly effective in the subject synthesis of MTBE are the synthetic MFI-type zeolites. Preferably said zeolites should be in a strongly acidic form whereby some or all of the cations (Group I or II, alkali or alkaline earth metals such as sodium, potassium, calcium or magnesium)) are exchanged by protons either through ammonium exchange followed by thermal stabilization (deammoniation or removal of ammonia) at elevated temperatures (for example 400xc2x0 C. to 500xc2x0 C.) or through mineral acid treatment, etc. The mineral acids may include hydrochloric acid, sulfuric acid or nitric acid.
The aluminum fluoride-modified zeolite is prepared by treating the said MFI-type zeolite with aluminum fluoride in the presence of distilled water. Preferably the aluminum fluoride is mixed with the distilled water in a sealed container and heated at elevated temperature before adding the zeolite to the said mixture of aluminum fluoride and distilled water.
In a further embodiment of the invention, the zeolite catalyst comprises 1 to 2 weight percent of the total reaction contents. In a further embodiment of the invention, the methanol comprises about 99 weight percent of methanol. In a further embodiment of the invention, the isobutene comprises about 98 weight percent of isobutene. In a further embodiment of the invention, methanol comprises about 41 weight percent of the reaction mixture. In a further embodiment of the invention, the isobutene comprises about 59 weight percent of the reaction mixture.
The said catalyst may be in the form of powders, pellets, granules, spheres, shapes and extrudates. The examples described herein demonstrate the advantages of using powder form.
The reaction may be carried out in either a stirred slurry reactor or in a fixed bed continuous flow reactor. The catalyst concentration should be sufficient to provide the desired catalytic effect.
Etherification can generally be conducted at temperatures from 40xc2x0 C. to 150xc2x0 C.; the preferred range is 40 to 100xc2x0 C. The total operating pressure may be from 1 bar to 66 bar, or higher. The preferred pressure range is about 1 bar to 33 bar.
The examples which follow illustrate the synthesis of MTBE from methanol and isobutene using aluminum fluoride MFI-type zeolite particularly in the form of powder. The examples are a means of illustration and it is understood that the invention is not meant to be limited thereby.
Conversion of the isobutene (Isobutene, mole %) is estimated in the following examples using the equations:                     (Moles of Isobutene in the Feed)            -              (Moles of Isobutene in the Product)                    Moles of Isobutene in the Feed        xc3x97  100
Selectivity of MTBE (MTBE, mole %) is estimated from the following equation:             Moles of MTBE in the Product                      (Moles of Isobutene in the Feed)            -              (Moles of Isobutene in the Product)              xc3x97  100
It may be noted that
i. Comparing etherification data in Table I and Example 5, using the aluminum-fluoride-modified zeolite ZCIC-10, prepared by the method of Example 1, with the data of ZCIC-10 alone (Comparative Example A, Table V) it is seen that the isobutene conversion levels with aluminum-fluoride-modified zeolite of Example 5 occur at all operating temperatures, but particularly at 80xc2x0 C. to 100xc2x0 C. are at least three times higher than for ZCIC-10 alone.
ii. Comparing etherification data in Table II and Example 6, using the aluminum-fluoride-modified zeolite ZCIC-10, prepared by the method of Example 2, with data of ZCIC-10 alone (Comparative Example A, Table V) it is seen that the isobutene conversion levels with aluminum-fluoride-modified zeolite of Example 6 occur at all operating temperatures, but particularly at 80 to 100xc2x0 C. are at least four times higher than for ZCIC-10 alone as well as higher than that of Example 5.
iii. Comparing etherification data in Table III and Example 7, using the aluminum-fluoride-modified zeolite ZCIC-10, prepared by the method of Example 3, with data of ZCIC-10 alone (Comparative Example A, Table V) it is seen that the isobutene conversion levels with aluminum-fluoride-modified zeolite of Example 7 occur at all operating temperatures, but particularly at 80 to 100xc2x0 C. are significantly and measurably at least five times higher than for ZCIC-10 alone as well as higher than those of Example 5 (Table I) and 6 (Table II).
iv. Comparing etherification data in Table IV and Example 8, using the aluminum-fluoride-modified zeolite MZ-25, prepared by the method of Example 4, with data of MZ-25 alone (Comparative Example B, Table VI) it is seen that the isobutene conversion levels with aluminum-fluoride-modified zeolite of Example 8 occur at all operating temperatures, but particularly at 80 to 100xc2x0 C. are significantly and measurably higher than for MZ-25 alone.
v. Comparing the etherification data in Table I and Example 5, using the aluminum-fluoride-modified zeolite ZCIC-10, prepared by the method of Example 1, with the data of aluminum-fluoride-modified zeolite MZ-25, prepared by the method of Example 4 and given in Table VI, it is seen that the isobutene conversion levels with aluminum-fluoride-modified zeolite of Example 5 occur at all operating temperatures are significantly and measurably higher than for aluminum-fluoride-modified zeolite MZ-25, prepared by the method of Example 4 and given in Table VI.
vi. Comparing the etherification data in Table V and Comparative Example A, using the ZCIC-10 zeolite, with the data of MZ-25 zeolite given in Table VI and Comparative Example B, it is seen that the isobutene conversion levels with ZCIC-10 zeolite given in Table V occur at all operating temperatures are significantly and measurably higher than for MZ-25 zeolite given in Table VI.